1. Field of the Invention
The present invention relates in general to a lithographic process for defining a pattern during fabrication of integrated circuits (ICs). More particularly, the invention provides a template and a method employing the template that allows an operator to check whether a mask pattern projected onto a wafer for IC production is in focus or not.
2. Description of Related Art
As the integration density of ICs increase, semiconductor fabrication takes place at the sub-micron very large scale integration (VLSI) level. Lithographic processes for pattern definition on a semiconductor wafer blank become more and more challenging. With the feature size of ICs reduced to sub-micron size (&lt;10.sup.-6 m), light used to expose an IC pattern on a blank wafer must have correspondingly shorter wavelengths, such as the g line of wavelength 465 nm, i line of wavelength 365 nm, or deep ultraviolet (DUV) of wavelength 248 nm. However it is difficult to use such wavelengths of light because of a focus problem. The depth of field of a mask image IC pattern focused onto a wafer from projection of a mask is significantly reduced. For example, consider the use of the KrF DUV as the exposure light in the fabrication of ICs of feature size 0.25 .mu.m. The depth of field that can be obtained is less than 0.8 .mu.m. Since the process latitude is dependent mainly on two factors: the critical dimension variation and the depth of field, more critical control of the focusing of the mask pattern on the wafer is required. A reduction in the critical dimension will sharply reduce the depth of field correspondingly. Therefore, focus control of a stepper for performing the lithographic process, becomes extremely critical.
The result of the lithographic process significantly affects the yield of IC fabrication. If the mask pattern definition image was not in proper focus the entire wafer must be discarded and the fabrication process started all over again on a new wafer.
FIG. 1 (Prior Art) is a schematic diagram of a conventional stepper performing a lithographic process for IC production. Light from an exposure light source 100 passes through an aperture 110 and a collimating lens 120 to a mask 130. Mask 130 has an IC pattern thereon which must be focused on a wafer 150. Light passing through the mask 130 is focused by a lens 140 to form an image of the mask pattern on the wafer 150.
Presently there is no setup standard for the stepper to check the precision of the focusing of the pattern definition image on the wafer. Also, it would be quite a laborious and time-consuming procedure to quantitatively obtain a measure of defocus of an out-of-focus image. Accordingly, defocus inspection is generally not carried out during IC production. Only when defocus significantly exceeds process latitude can quality control personnel recognize the defective condition. This lack of appropriate measures for timely inspecting whether a mask pattern image is in focus or not adversely affects quality control of the IC chip production. Poorly focused mask patterns don't become evident until final testing of the IC. If an IC is bad due to poor focus, many process steps are wasted.